docs: def _soi(self, orb):
          """Evaluate the need for SOI transition, by comparing the radial distance
          between the considered body and the spacecraft
  
          If therer is no body in sight, default to central body.
          """
  
          for body in self.alt:
              soi = self.SOI[body.name]
              sph = orb.copy(frame=soi.frame, form='spherical')
              if sph.r < soi.radius:
                  active = body
                  break
          else:
              active = self.central
  
          return active